The present invention is directed to an apparatus and method for preparing high aspect ratio articles by vapor deposition. More specifically, the present invention is directed to an apparatus and method for preparing high aspect ratio articles by vapor deposition on male mandrels.
Chemical vapor deposition (CVD) has been used to provide various materials either in freestanding bulk form or as a layered coating on a substrate. CVD methods of providing zinc sulfide layers and articles are described in commonly assigned U.S. Pat. Nos. 4,978,577 and 5,686,195, and in references cited therein. Generally, the previous methods of producing bulk material by CVD processing have provided solid shapes, the surfaces of which are then machined to their final article form. Replication techniques for producing “near-net shape parts” which minimize machining are suggested and some results described in Goela et al., “CVD Replication for Optics Applications”, SPIE Proc., 1047, pages 198-210 (1989). A replication process for producing silicon carbide articles described in U.S. Pat. No. 4,997,678 provides an in situ applied coating of carbon on a polished pre-shaped substrate prior to initiating the chemical vapor deposition of silicon carbide to form an article replicating a surface of the substrate.
While the techniques described in the above references have produced near-net shape articles, there continues to be a need for a precision replication technique which will provide precisely shaped articles, such as optical components, without the need for final machining of the optical surfaces. The techniques described in U.S. Pat. No. 4,997,678 provide relatively good replicas, however, the carbon rich film used therein is applied in situ at the beginning of each production deposit. Such does not provide an opportunity to measure or otherwise verify the dimensions of the film prior to commencing the production run, and allows uneven growth of the carbon rich film to result in uncontrolled deviations in the replicated article. Moreover, the carbon rich film tends to adhere to the surface of the replica when such is separated from the substrate. These characteristics tend to limit the ability of this technique to replicate the finish of the substrate.
Techniques that provide precision replication of, and facile release from, a substrate are particularly needed. For instance, infrared sensors used for navigation, guidance and targeting on aircraft and missiles require protection from the elements by transmissive windows or domes. Preferably such windows or domes are provided in a shape which minimizes aerodynamic drag while avoiding transmission irregularities. In a missile, the preferred location is in the nose. A spherical dome at this location produces considerable drag which can be significantly reduced by factors of two or more by the use of an extended generally conical shaped dome. The fabrication of such a dome, however, is made difficult, or even impossible, when machining of the interior surfaces of the dome is required since the functionality of the required apparatus decreases as the diameter of the cone decreases and its length is extended. The precision replication of such interior surface of a dome on a reusable mandrel, such that machining or polishing of such surface is not required, would provide significant fabrication advantages. Certain CVD produced bulk materials, such as zinc sulfide and zinc selenide, however, due to their thermal expansion characteristics and those of their preferred mandrel materials, have not previously been produced on the exterior surfaces of a curved male mandrel. Instead when such materials have been used to produce curved products, they have been fabricated by deposits formed on the interior surfaces of curved female mandrels, whereby the interior surfaces of such curved articles require considerable machining to provide their required final figure and finish.
Production of zinc sulfide and zinc selenide shaped parts such as domes is described in U.S. Pat. No. 5,453,233 to Teverosky et al. In this method, the material is deposited on a female mandrel as shown in FIG. 2. FIG. 2 shows a schematic of an isolation fixture 30 with multiple female mandrels 32. Reactants 34 pass from a gas injector 36 toward exhaust 38. Each mandrel 32 is fabricated with a figure that is negative of the actual part being produced. In this configuration, the outer surface of the dome is produced in a near-net shape by the CVD process. For this configuration to succeed, the thermal expansion coefficient of the mandrel preferably is less than that of the material being deposited. The female configuration is preferred over the male configuration because the mandrel does not stick out in the flow of reactants 34 and convection currents 40 effecting the flow pattern in the deposition area. The method is very effective in producing domes of aspect ratios (AR)<0.5. The aspect ratio, which is also known as fineness ratio, is defined as the ratio of the dome length to its diameter. See also “Replication of Conformal Surfaces by Chemical Vapor Deposition” by Goela et al., page 315, lines 2-3, presented at the U.S. Air Force Academy on Apr. 24-27, 2000.
If an AR of equal to or greater than 0.5 is desired, the above method can be used to produce zinc sulfide domes by the CVD process but the above method does not yield domes of satisfactory quality. This is due to the following reasons: (1) the thickness of the material deposited at the dome base is larger than at the apex reducing the effective diameter of the opening in the female mandrel; (2) the thicker deposit at the base makes it difficult to remove the deposit from the isolation fixture; (3) the quality of the material deposited at the dome apex gets degraded when the deposition thickness is large. Accordingly, better methods are desired to deposit high quality shaped parts of high aspect ratios.
U.S. Pat. No. 6,042,758 to Goela and assigned to CVD, Inc. discloses a process for preparing dimensionally precise articles by chemical deposition. The '758 patent discloses that a female mandrel is preferred for preparing articles with high aspect ratios (column 4, lines 1-2). The ratio as defined in the '758 patent is the ratio of the article's diameter to its height (or length) (column 4, lines 3-4). Male mandrels are preferred when an article with an aspect ratio of less than 2 is desired (column 4, lines 4-5). When the aspect ratio is determined by length to diameter, the aspect ratio is less than 0.5. However, the '758 patent does not address the problems associated with preparing domes having aspect ratios greater than 0.5 as discussed above. The '758 patent also does not disclose the orientation of mandrels in a vapor deposition chamber to obtain high aspect ratio domes.
A paper by Goela et al. Entitled “Fabrication of Conformal ZnS Domes by Chemical Vapor Deposition” presented at the SPIE meeting at Orlando, Fla. on Apr. 5-9, 1999 and on May 11-13, 1999 discloses domes made of ZnS with high aspect ratios using male mandrels. Although the paper discloses the process conditions and the material composing the mandrels, the paper is silent on mandrel orientation in the deposition chamber. An earlier paper also by Goela et al. Entitled “Precision Replication of Conformal ZnS Optics” disclosed at the 7th DOD Electromagnetic Window Symposium at APL, Laurel, Md., on May 5-7, 1998 discloses employing articles made on male mandrels. Process conditions for vapor depositing ZnS on the mandrels are disclosed, but not mandrel orientation in the deposition chamber.
FIG. 3 shows a schematic of chemical vapor deposition by impinging flow. In the impinging flow schematic, both the male and female mandrels can be used and the flow of gases is parallel to the dome axis. The parallel orientation of the domes reduces the available deposition area, i.e. fewer mandrels can be placed in the deposition chamber. Placing several mandrels in stages can increase the deposition area. However, this reduces deposition thickness considerably from one stage to the next due to reagent depletion effects. Domes with very large thickness variations are produced. Also fabrication time is increased, and material yield is reduced. Thus this method is cost ineffective. FIG. 3 shows male mandrels 42 on isolated fixtures 46 in a deposition chamber 44 in a two-stage impinging flow configuration. Gas flow 48 has to negotiate several bends to move from one stage to the other stage. Such bending depletes the reagents at a faster rate and increases the thickness non-uniformity of the domes from stage to stage. Accordingly, there is a need for a method and apparatus with an improved mandrel orientation to prepare domes with high aspect ratios.
Another problem associated with vapor deposition on a mandrel is cracking of the article when it is removed from the mandrel. Cracks or flaws can form at structural weak points in the article during chemical vapor deposition. Such flaws are common along the flange portion of domes formed at the base of the mandrel. Such flaws can lead to the cracking of the dome when it is removed from the mandrel. Loses of deposited articles can be costly to the industry. Accordingly there also is a need for a method of preventing unwanted cracking of deposited articles.
A primary object of the present invention is to provide an apparatus and method that enables the formation of high aspect ratio domes.
Another object of the present invention is to provide an apparatus and method that prepares high aspect ratio domes employing male mandrels.
A further object of the present invention is to provide a method of preparing high aspect ratio domes where the deposit is thicker at the apex than the base of the dome.
An additional object of the present invention is to provide a method of making high aspect ratio domes such that the domes do not crack when the domes are removed from the mandrels.
Other objects and advantages of the present invention will be apparent to those of skill in the art by reading the following description and the appended claims.